Automotive parts such as a gear, a transmission, and a seat recliner have been commonly produced by cold-working a hot-rolled steel sheet that is a carbon steel for machine structural use according to JISG4051 into a desired shape and subsequently performing a quenching treatment in order to achieve desired hardness. Accordingly, the hot-rolled steel sheets used as a material of such automotive parts have been required to have high cold formability and high hardenability, and various steel sheets have been proposed.
For example, Patent Literature 1 discloses a steel for machine structural use having high cold formability and high resistance to decarburization, the steel having a composition containing, by massa, C: 0.1% to 1.2%, Si: 0.01% to 2.5%, Mn: 0.1% to 1.5%, P: 0.04% or less (including 0%), S: 0.0005% to 0.05%, Al: 0.2% or less, one or two elements selected from Te: 0.0005% to 0.05% and Se: 0.0005% to 0.05%, N: 0.0005% to 0.03%, the total content of S and the one or two elements selected from Te and Se being 0.005% to 0.05%, and the balance being Fe and incidental impurities, the steel having a microstructure mainly composed of ferrite and pearlite, the crystal grain size number of ferrite according to JIS G 0552 being 11 or more. Patent Literature 1 also discloses a steel for machine structural use having a composition containing, in addition to the above-described constituents, Sb: 0.001 to 0.05%; a steel for machine structural use having a composition containing, in addition to the above-described constituents, one or more elements selected from Cr: 0.2% to 2.0%, Mo: 0.1% to 1.0%, Ni: 0.3% to 1.5%, Cu: 1.0% or less, and B: 0.005% or less; a steel for machine structural use having a composition containing, in addition to the above-described constituents, one or more elements selected from Ti: 0.002% to 0.05%, Nb: 0.005% to 0.1%, and V: 0.03% to 0.3%; and a steel for machine structural use having a composition containing, in addition to the above-described constituents, one or more elements selected from Mg: 0.0002% to 0.01%, Zr: 0.0001% to 0.01%, and Ca: 0.0002% to 0.008%. Patent Literature 1 further discloses a method for producing a steel for machine structural use having high cold formability and high resistance to decarburization, in which a steel having the above-described composition is hot-rough-rolled at 850° C. or more and 1000° C. or less, finish-rolled at 700° C. or more and 1000° C. or less, cooled to 500° C. or more and 700° C. or less at a cooling rate of 0.1° C./sec or more and less than 5° C./sec, immediately maintained in a furnace having an atmosphere temperature of 650° C. or more and 750° C. or less for 15 minutes or more and 90 minutes or less, and is allowed to cool.
Patent Literature 2 discloses a high-carbon steel sheet with high formability, high hardenability, high weldability, high resistance to carburization, and high resistance to decarburization, the steel having a composition containing, by mass %, C: 0.2% to 0.35%, Si: 0.03% to 0.3%, Mn: 0.15% to 1.2%, Cr: 0.02% to 1.2%, P: 0.02% or less, S: 0.02% or less, Mo: 0.2% or less, Ti: 0.01% to 0.10%, B: 0.0005% to 0.0050%, and one or more elements selected from Sn, Sb, Bi, and Se such that the total content of the one or more elements is 0.0003% to 0.5% or a composition containing, in addition to the above-described constituents, one or more elements selected from Ce: 0.05% or less, Ca: 0.05% or less, Zr: 0.05% or less, and Mg: 0.05% or less. Patent Literature 2 also discloses a method for producing a high-carbon steel sheet with high formability, high hardenability, high weldability, high resistance to carburization, and high resistance to decarburization, in which a steel having the above-described composition is hot-rolled with a finishing temperature of Ar3+10° C. to Ar3+50° C. and a coiling temperature of 550° C. to 700° C. and subsequently pickling is performed.
Patent Literature 3 discloses a high-carbon hot-rolled steel sheet having a composition containing, by mass %, C: 0.15% to 0.37%, Si: 1% or less, Mn: 2.5% or less, P: 0.1% or less, S: 0.03% or less, sol. Al: 0.1% or less, N: 0.0005% to 0.0050%, B: 0.0010% to 0.0050%, at least one element selected from Sb and Sn: 0.003% to 0.10% in total, and the balance being Fe and incidental impurities, in which 0.50 (14[B])/(10.8[N]) is satisfied, the high-carbon hot-rolled steel sheet having a microstructure including a ferrite phase and cementite, the average grain size of the ferrite phase being 10 μm or less, the spheroidizing ratio of the cementite being 90% or more, where [B] and [N] refer to the contents (mass %) of B and N, respectively. Patent Literature 3 also discloses a high-carbon hot-rolled steel sheet having a composition containing, in addition to the above-described constituents, at least one elements selected from Ti, Nb, and V: 0.1% or less in total; and a high-carbon hot-rolled steel sheet having a composition containing, in addition to the above-described constituents, at least one elements selected from Ni, Cr, and Mo: 1.5% or less in total. Patent Literature 3 further discloses a method for producing a high-carbon hot-rolled steel sheet, in which a steel having the above-described composition is hot-rolled with a finishing temperature of the Ar3 transformation temperature or more, cooled to a cooling stop temperature of 550° C. to 650° C. within 10 s, coiled at a coiling temperature of 500° C. to 650° C., pickled, and annealed at 640° C. or more and the Ac1 transformation temperature or less in order to spheroidize cementite. Patent Literature 3 also discloses a method for producing a high-carbon hot-rolled steel sheet, in which a steel having the above-described composition is hot-rolled with a finishing temperature of the Ar3 transformation temperature or more, cooled from 650° C. or more to a cooling stop temperature of 450° C. to 600° C. at an average cooling rate of 50° C./s or more, coiled within 3 s after being cooled, pickled, and annealed at 640° C. or more and the Ac1 transformation temperature or less in order to spheroidize cementite.
In the above-described steel sheets, the hardenability of the steel sheet is enhanced by using elements such as Mn, P, B, Cr, Mo, and Ni. For example, it is described in the technique disclosed in Patent Literature 3 that elements such as Mn, P, and B enhance the hardenability of a steel sheet.